JC virus (JCV), an opportunistic pathogen, infects >80% of the world's population. In immunodeficient individuals, a primary or reactivated JCV infection may result in the fatal demyelinating brain disease, PML, (progressive multifocal leukoencephalopathy). Once considered rare, this emerging disease now claims the lives of ~5% of AIDS patients. During the past 7 years convincing evidence has been uncovered linking JCV to specific human tumors. Our ultimate goal is to define JCV's pathogenic and oncogenic mechanisms and to identify strategies for immunomodulation of disease progression. We propose that a key to unraveling such mechanisms lies in a structural and functional dissection of the unexpectedly complex viral early coding region. Five JCV tumor proteins are translated from transcripts alternatively spliced from a single mRNA. In addition to large and small tumor antigens (TAg, tAg), 3 truncated versions of TAg, T'135, T'136 and T'165, are produced. Although the structures of T'proteins suggest they specify functional domains critical to viral lytic and transforming behaviors, study of TAg isoforms has been all but ignored in favor of focusing upon TAg and tAg. We hypothesize a complete understanding of JCV's pathogenic and oncogenic potential requires us to examine unique interactions of T'proteins with viral and cellular targets, to identify activities of T'proteins that complement, replace or antagonize TAg-mediated functions and to determine mechanisms by which T'protein expression is regulated in different cellular contexts. The aims designed to test our hypothesis are: 1) To identify unique and shared oncogenic functions of T'165, T'136 and T'135. We hypothesize that T'proteins contribute to JCV oncogenic activity in vitro and in vivo. 2) To investigate pro- and anti-apoptotic activities of JCV tumor proteins. We hypothesize JCV TAg induces apoptosis, an activity blocked by expression of at least 1 other early protein. 3) To unravel mechanisms by which differential expression of JCV tumor proteins is regulated through alternative splicing of the early pre-mRNA. We hypothesize differential expression of JCV tumor proteins in infected vs. transformed cells signals temporal and cell-type specific regulation of alternative splicing. Relevance: This work will contribute to our understanding of the oncogenic and pathogenic activities of JCV, a human virus that causes a fatal brain disease in AIDS patients and, recently, in multiple sclerosis and Crohn's patients treated with the drug Tysabri.